Aqueous surfactant solutions, particularly those used in the field of personal care as hair shampoos, foam baths, shower baths, hand washing pastes and the like, mostly contain anionic surfactants, for example alkyl ether sulfates. In order to stabilize these clear or disperse systems and to make them easier for the user to handle, thickeners are normally added to the surfactant solutions. Various inorganic and organic compounds used to increase the viscosity of anionic surfactant solutions are known to the expert. Water-soluble electrolyte salts, typically sodium chloride, are generally used as inorganic thickeners. Examples of organic thickeners are fatty acid alkanolamides, polyethylene glycol difatty acid esters and a number of water-soluble polymers. In most cases, the required viscosity of the surfactant solution can be adjusted with inorganic electrolyte salts alone, but only by using large quantities. Accordingly, organic thickeners are generally used in addition to the inorganic salts although, in some cases, they are attended by a number of disadvantages. Thus, surfactant solutions thickened with polyethylene glycol fatty acid diesters often show inadequate viscosity stability in storage while water-soluble polymers show unwanted slimy flow behavior with a tendency to become stringy in the thickened surfactant solutions.
Accordingly, it is proposed in German patent applications DE-A-37 30 179 and DE-A-38 17 415 to use addition products of ethylene oxide and/or propylene oxide, optionally with a narrow homolog distribution, with saturated and/or unsaturated fatty alcohols for thickening surfactant solutions. These products are not attended by the disadvantages mentioned above. However, there is a need for further organic thickeners with an increased thickening effect which make it possible in particular to use low contents of organic and inorganic thickeners for a given viscosity to be adjusted in the surfactant solution.
It has now surprisingly been found that this problem can be solved by using fatty acid esters of alkylene glycols as thickeners.